bitflipper
I would take a finished mix of their best song and import it into a track in the new project. That would give you a handy reference to compare/analyze with a quick click of the solo button. Throw an instance of SPAN on it, and another on your master. Put both into Mastering mode, which is slower and better-suited for a broader overview.
Don't bother with actual EQ-matching; that'll likely just waste time. All you're trying to do is avoid gross differences, such as a mix that's clearly bass-heavy compared to the other finished tracks.
+1 to this, I'd also not bother with eq matching because what you record with them will differ from what they recorded with Matt. When you grab a mastered curve from something that is not what your source is NOW, it will not sound right. For example, you know I've been working on some old Van Halen songs for my tribute band. The only way the VH master curve would somewhat work for me is if I went out of my way to cop the instrumentation and the mix as close as possible. Even there, I'd still probably fall short.
Listen to what they have and try to concentrate on the instrumentation choices they made as well as how the instruments were enhanced/mixed. Then do your best to uniform if you can. If not, it is what it is brother. You do the best you can and make the song sound the best that it can be as an entity instead of trying to compete with what is already there. If the instruments used for the other recordings will not be the same instrument sounds used on the recording they do with you, you're already at a disadvantage.
Or, who knows...with your instrumentation, you may wind up with a better product which is always a plus too. I'd rather have someone say "dude, this is actually better than what we did with the other songs...how can you make it sound a bit worse?" LOL! You just never know man...stranger things have happened in this field....and sometimes, famous producers/engineers (eventhough they know their stuff) get famous due to the band they work with. That band would remain famous even if you me or bitflipper did the mix. When fans love a band, that band is going to sell no matter who engineers, mixes or masters them. So you just may surprise yourself here Chuck. Just do what you do the best that you can brother.
At any rate, I wish you the best of luck with this. I got faith in you man! :)